/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package io.rsocket.internal.jctools.util;

import static io.rsocket.internal.jctools.util.UnsafeAccess.UNSAFE;

/**
 * A concurrent access enabling class used by circular array based queues this class exposes an
 * offset computation method along with differently memory fenced load/store methods into the
 * underlying array. The class is pre-padded and the array is padded on either side to help with
 * False sharing prvention. It is expected theat subclasses handle post padding.
 *
 * <p>Offset calculation is separate from access to enable the reuse of a give compute offset.
 *
 * <p>Load/Store methods using a <i>buffer</i> parameter are provided to allow the prevention of
 * final field reload after a LoadLoad barrier.
 *
 * <p>
 *
 * @author nitsanw
 */
@InternalAPI
public final class UnsafeRefArrayAccess {
  public static final long REF_ARRAY_BASE;
  public static final int REF_ELEMENT_SHIFT;

  static {
    final int scale = UnsafeAccess.UNSAFE.arrayIndexScale(Object[].class);
    if (4 == scale) {
      REF_ELEMENT_SHIFT = 2;
    } else if (8 == scale) {
      REF_ELEMENT_SHIFT = 3;
    } else {
      throw new IllegalStateException("Unknown pointer size: " + scale);
    }
    REF_ARRAY_BASE = UnsafeAccess.UNSAFE.arrayBaseOffset(Object[].class);
  }

  /**
   * A plain store (no ordering/fences) of an element to a given offset
   *
   * @param buffer this.buffer
   * @param offset computed via {@link UnsafeRefArrayAccess#calcElementOffset(long)}
   * @param e an orderly kitty
   */
  public static <E> void spElement(E[] buffer, long offset, E e) {
    UNSAFE.putObject(buffer, offset, e);
  }

  /**
   * An ordered store(store + StoreStore barrier) of an element to a given offset
   *
   * @param buffer this.buffer
   * @param offset computed via {@link UnsafeRefArrayAccess#calcElementOffset}
   * @param e an orderly kitty
   */
  public static <E> void soElement(E[] buffer, long offset, E e) {
    UNSAFE.putOrderedObject(buffer, offset, e);
  }

  /**
   * A plain load (no ordering/fences) of an element from a given offset.
   *
   * @param buffer this.buffer
   * @param offset computed via {@link UnsafeRefArrayAccess#calcElementOffset(long)}
   * @return the element at the offset
   */
  @SuppressWarnings("unchecked")
  public static <E> E lpElement(E[] buffer, long offset) {
    return (E) UNSAFE.getObject(buffer, offset);
  }

  /**
   * A volatile load (load + LoadLoad barrier) of an element from a given offset.
   *
   * @param buffer this.buffer
   * @param offset computed via {@link UnsafeRefArrayAccess#calcElementOffset(long)}
   * @return the element at the offset
   */
  @SuppressWarnings("unchecked")
  public static <E> E lvElement(E[] buffer, long offset) {
    return (E) UNSAFE.getObjectVolatile(buffer, offset);
  }

  /**
   * @param index desirable element index
   * @return the offset in bytes within the array for a given index.
   */
  public static long calcElementOffset(long index) {
    return REF_ARRAY_BASE + (index << REF_ELEMENT_SHIFT);
  }
}
